The NMDA receptor which is one of glutamate receptors is found on the nerve cell membranes in the brain and involved in various neurophysiologic events including the plasticity of nerves, as well as cognition, attention, and memory. The NMDA receptor has a plurality of allosteric binding sites, among which is the glycine binding site (NMDA receptor complex glycine binding site). It has been reported that the NMDA receptor complex glycine binding site takes part in the activation of the NMDA receptor (Non-Patent Document 1).
An action potential arriving at the presynaptic terminal of a glycinergic nerve triggers the release of glycine into the synaptic cleft. The released glycine binds to the postsynaptic receptor or the like and is thereafter carried away by the transporter to leave the synaptic cleft. Hence, it is postulated that the glycine transporter regulates the function of the NMDA receptor through regulation of the glycine level in the extracellular fluid.
The glycine transporter (GlyT) is a protein involved in the reuptake of extracellular glycine into the cell and two subtypes, GlyT1 and GlyT2, have so far been identified. GlyT1, which mainly develops in the cerebral cortex, hippocampus, thalamus, etc., has been reported to be associated with such diseases as schizophrenia, Alzheimer's disease, cognitive dysfunction, dementia, anxiety disorders (generalized anxiety disorder, panic disorder, obsessive-compulsive disorder, social anxiety disorder, posttraumatic stress disorder, specific phobia, acute stress disorder, etc.), depression, drug addiction, spasm, tremor, pain and sleep disorder (Non-Patent Documents 2-4).
Compounds featuring the GlyT1 inhibiting action and having a 5-membeed cyclic heteroarylamide structure have been reported in the following documents (Patent Documents 1-3 and Non-Patent Documents 5 and 6).